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4 points by waterhouse 3351 days ago | link | parent

Egad, you are thoroughly right and I have rewritten the post. mentally smacks myself on the head Thank you, that is a much better implementation. I'm a little bit disappointed that it's so easy...

Uh-oh, the edit window has timed out on my post, and I've noticed that my version doesn't do any type-checking. That is el baddo. So far, it hasn't caused things to crash yet...

  arc> (= x 'meh)
  meh
  arc> (= (car x) 2)
  2
  arc> x
  meh
...but that's just luck and I wouldn't rely on it. [It does destroy the world when you set the car of a table.] Better version is here:

  (require racket/unsafe/ops)
    
  (define x-set-car!
    (let ((fn (namespace-variable-value 'set-car! #t (lambda () #f))))
      (if (procedure? fn)
          fn
          (lambda (p x)
            (if (pair? p)
                (unsafe-set-mcar! p x)
                (raise-type-error 'set-car! "pair" p))))))
  (define x-set-cdr!
    (let ((fn (namespace-variable-value 'set-cdr! #t (lambda () #f))))
      (if (procedure? fn)
          fn
          (lambda (p x)
            (if (pair? p)
                (unsafe-set-mcdr! p x)
                (raise-type-error 'set-cdr! "pair" p))))))


1 point by waterhouse 3351 days ago | link

Man, I really wish I could continue editing my original post, 'cause without the type-checking, it's kind of a nuclear bomb in case someone accidentally uses it on a table. Perhaps I should just have left the post unchanged, or just added "Edit: Look in the comments for improvements" at the bottom.

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4 points by akkartik 3351 days ago | link

I still have the edit link (because I can kill stories). Feel free to email me an updated version (address in profile) and I'll replace it for you.

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1 point by waterhouse 3351 days ago | link

Excellent. You've got mail.

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